Ab initio study of metallic aluminum hydrides at high pressures

Metallic phases of ${\mathrm{Al}}_2\mathrm{H}$ and AlH at megabar pressures are predicted to be possible by using ab initio density functional calculations. The ${\mathrm{Al}}_2\mathrm{H}$ phase is stabilized above 155 GPa, where several candidate structures are quite competitive; the structural properties suggest that ${\mathrm{Al}}_2\mathrm{H}$ has the phase where Al atoms form an hcp structure and H atoms occupy the octahedral sites in a random manner. The AlH phase is stable above 175 GPa, where the structure takes $R\overline{3}m$ symmetry. Superconducting transition temperature ($T_c$) of the ${\mathrm{Al}}_2\mathrm{H}$ phase is estimated to be of the order of 1 K. In contrast, $T_c$ of the $R\overline{3}m$ AlH reaches 58 K at 180 GPa. The electronic structures around the Fermi energy in the $R\overline{3}m$ AlH are insensitive to pressure compared with those in the well-known $Pm\overline{3}n$ phase of ${\mathrm{AlH}}_3$. Accordingly, while theoretical $T_c$ of the $Pm\overline{3}n{\mathrm{AlH}}_3$ rapidly decreases with compression and becomes almost zero above $\sim 200$ GPa, that of the $R\overline{3}m$ AlH remains to be 21 K even at 335 GPa. This means that although superconductivity was not observed experimentally in ${\mathrm{AlH}}_3$, it might be achieved in AlH.

Figure 1

(a) Relative enthalpy per atom of ${\mathrm{Al}}_{1-x}{\mathrm{H}}_x$ as a function of $x$ for $0\le x\le 1$ without ZPE, where stable compositions are connected with solid lines; (b) for $0\le x\le 1$ with ZPE; (c) for $0\le x\le 0.75$ without ZPE; (d) for $0\le x\le 0.75$ with ZPE. (e) Comparison of enthalpy per atom between candidate structures in ${\mathrm{Al}}_2\mathrm{H}$ with ZPE; (f) in AlH. The enthalpy of Al is calculated for fcc, hcp, and bcc structures [29, 30, 31], and that of H for the $C2/c$ [32], the $Cmca$-12 [32], the $Cmca$ [33], and the Cs-IV ($I{4}_1/amd$) [34] structures. Structural parameters of the candidate structures are summarized in Ref. [23].

Figure 2

The lowest-enthalpy structures of ${\mathrm{Al}}_2\mathrm{H}$ and AlH [23].

Figure 3

Radial distribution functions of the candidate structures in ${\mathrm{Al}}_2\mathrm{H}$ at $r_s=1.62$ (left) and AlH at $r_s=1.57$ (right), where a broadening parameter is set at 0.1 Å. See Ref. [35] for the definition of $r_s$.

Figure 4

Partial DOS (PDOS) of the $P\overline{3}m1{\mathrm{Al}}_2\mathrm{H}$ at 195 GPa and the $R\overline{3}m$ AlH at 215 GPa.

Figure 5

Phonon dispersion relation, where the areas of the circles on the dispersion curves denote the magnitude of partial electron-phonon coupling parameter ${\lambda }_{\nu }\left(\mathbf{q}\right)$ [14]: (a) the $P\overline{3}m1{\mathrm{Al}}_2\mathrm{H}$ at 195 GPa, (b) the $R\overline{3}m$ AlH at 215 GPa, and (c) the $Pm\overline{3}n{\mathrm{AlH}}_3$ at 105 GPa.

Figure 6

Pressure dependencies of the DOS (top two panels) and the Eliashberg function ${\alpha }^{2}F$ multiplied by $2/\omega$ (bottom two panels) in the $R\overline{3}m$ AlH and the $Pm\overline{3}n{\mathrm{AlH}}_3$.